A popular agricultural product is canned peach halves, slices and cubes. The peach variety typically used for canning is referred to as a "cling" peach, whereas the popular eating peach variety is referred to as "the free stone" peach, which is not used for canning because they lose their taste during the canning process. The variety names cling and free stone imply the relative ease with which the stone (hereafter "pit") can be removed from the fruit.
Many peach processors employ an Atlas splitting machine to remove the pit. This machine consist of a circumferential knife, that looks and functions much like the iris of a camera lens. As the blades of the machine close down on the peach, it cuts through the flesh until it meets the hard core of the pit. Once the pit is secured firmly in place by means of the blade, two cups approach from either side to grab the two peach halves. When the cups are in place they are rotated in opposite directions to twist the peach halves apart and separate them from the secured pit. Unfortunately, the blade cannot always adequately secure the pit and when the peach halves fall away, the entire pit may stay embedded in one of the halves. Alternatively, the pit may split in half or fragment into smaller pieces.
Successful removal of pits from cling peaches presents a considerable agricultural processing challenge. In conventional agricultural processing plants, split peach halves are visually inspected for pits or pit fragments by large numbers of inspectors standing on opposite sides of conveyors belts used to transport the peach halves. Unfortunately, the pit color closely matches the color of peach flesh. This is due in part to tendrils of peach flesh that cling to the surface of the pit. Therefore, the inspectors must rely on their visual shape recognition capabilities to recognize unacceptable product. Moreover, the inspectors often have to manually detect small "hidden" pit fragments by wiping the tip of their fingers around the cavity left in the peach by a removed pit. These inspection difficulties have previously ruled out automatically inspecting peach halves with machine vision techniques that detect visual wavelengths of light.
Even if machine vision inspection techniques were employed, the close color match between peach flesh and pits and the hidden nature of many pit fragments would render such inspection unreliable. Improving machine vision inspection reliability involves careful attention to both the camera or cameras employed and the illumination of the product being inspected. Suitable illumination typically employs a uniform, shadowless, high intensity light source to illuminate the product being inspected. Prior light sources include fluorescent lamps, incandescent bulbs, and short and long arc discharge lamps. The assignee of this application, SRC Vision, of Medford, Oreg. has used all of these sources and found them wanting in one aspect or another.
For example, FIG. 1 shows a "Brite-Lite" illumination source 10 manufactured by the assignee of this application, in which a fluorescent tube 12 is mounted at one foci of an elliptical or parabolic reflector 14 and the other foci lies in a linear inspection zone 16 on the plane of a conveyor belt 18 moving articles 20 to be inspected. A line scanning inspection camera 22 has its field of view that is co-aligned with the energy from fluorescent tube 12 focused in inspection zone 16 to maximize the amount of illumination reflected off articles 20 and received by inspection camera 22. This illumination technique produces a fairly uniform illumination inspection zone 16, but the illumination decreases near the edges of belt 18 because light illuminating the center of belt 18 propagates from any and all points along the length of fluorescent tube 12. However, because fluorescent tube 12 has a finite length and extends only five or six inches beyond the belt edges, illumination reaching points near the belt edges propagates mainly from portions of fluorescent tube 12 directly over the belt and, to a lesser extent, from any short portions that extend beyond the belt edges. Moreover, this technique is not entirely shadowless, which makes pit fragment detection difficult. Consider an article with some height, such as an apple cube lying within inspection zone 16. A point lying immediately to one side of the cube will receive light from only that portion of fluorescent tube 12 that extends in a direction away from that side of the cube. The cube itself will block the light from that portion of fluorescent tube 12 that extends in the direction of the cube. There is, however, some partial filling in of the shadow by that portion of fluorescent tube 12 that is not blocked by the cube.
To provide shadowless illumination, the light rays should ideally be parallel and perpendicular to the surface of belt 18. One way to produce this ideal illumination is to employ an illumination point source at an infinite distance. However, this technique is impractical because the illumination intensity decreases inversely with the square of the distance from the light source.
FIG. 2 shows another exemplary illumination source 30 that employs multiple incandescent lamps 32 each having an associated reflector. Illumination source 30 simulates multiple illumination point sources propagating from a significant distance, but is not very energy efficient because the illumination from each of lamps 32 is spread over a relatively large area of belt 18. Illumination uniformity is approximated by appropriately aiming lamps 32 and by adjusting their individual illumination levels. This is a labor intensive process that is prone to errors. Moreover, indiscriminate adjustment of lamp 32 illumination levels may alter their spectral wavelength distributions.
FIG. 3 shows yet another exemplary illumination source 40 that employs a pair of moderate length high-intensity discharge ("HID") tubes 42 positioned at the foci of two astigmatic cylindrical projection lenses 44. In illumination source 40, only those light rays that intersect flat back surfaces 46 of projection lenses 44 are focused on inspection zone 16 of conveyor belt 18, which renders this technique inefficient. Moreover, because the lengths of HID tubes 44 is short compared to the width of belt 18, the light rays must diverge to spread across the width of belt 18, which introduces shadowing because the angle of incidence of the light rays is not perpendicular to belt 18. Using multiple HID lamps 44 and projection lenses 44 can somewhat alleviate this problem.
What is needed, therefore, is an illumination and detection technique suitable for automatically inspecting peach halves for pits and pit fragments.